Add NSG extension to merge SecurityRules

[super-harsh]

Closes #3050

What this PR does / why we need it:

This PR includes an extension to merge NSGSecurityRules into NSG resources. Also, as part of this PR had to add SecurityRules as an embedded resource within NSG to make sure the merge is possible.

If applicable:

• this PR contains tests

[matthchr]

Was this preserved?

[super-harsh]

Yes it is here

[codecov-commenter]

Codecov Report

Merging #3121 (df083f1) into main (7131774) will increase coverage by 0.03%.
The diff coverage is 64.17%.

@@            Coverage Diff             @@
##             main    #3121      +/-   ##
==========================================
+ Coverage   54.14%   54.18%   +0.03%     
==========================================
  Files        1418     1419       +1     
  Lines      608196   608049     -147     
==========================================
+ Hits       329328   329450     +122     
+ Misses     224867   224607     -260     
+ Partials    54001    53992       -9     

Impacted Files	Coverage Δ
...01101/network_security_group_spec_arm_types_gen.go	33.33% <ø> (ø)
.../v1api20201101/network_security_group_types_gen.go	59.35% <ø> (-0.47%)	⬇️
...ecurity_groups_security_rule_spec_arm_types_gen.go	33.33% <ø> (ø)
...0201101storage/network_security_group_types_gen.go	55.17% <ø> (ø)
...01101/network_security_group_spec_arm_types_gen.go	33.33% <ø> (ø)
...v1beta20201101/network_security_group_types_gen.go	56.52% <ø> (-0.63%)	⬇️
...ecurity_groups_security_rule_spec_arm_types_gen.go	33.33% <ø> (ø)
...0201101storage/network_security_group_types_gen.go	52.98% <ø> (-1.65%)	⬇️
...customizations/network_security_group_extension.go	53.57% <53.57%> (ø)
...twork/customizations/virtual_network_extensions.go	57.69% <63.33%> (+3.84%)	⬆️
... and 5 more

... and 33 files with indirect coverage changes